An emboldened Muslim Brotherhood appeared to be on a collision course with Egypt's ruling generals last night, as it called on its millions of supporters to protest against this week's military power grab and the recent dissolution of the Islamist-led parliament.

The call, which followed the Brotherhood's disputed claims of victory in the final round of presidential elections, came as Egypt's unelected army rulers faced further pressure from Washington yesterday.

Responding to what commentators have called a "soft coup" – which saw the military carve up substantial new powers for itself via an executive decree – officials in the US said they would review more than £1bn in annual military subsidies unless there was a swift handover to civilian rule.

The comments reflect the manner in which Washington's alliance with Cairo, for years a keystone of America's Middle East diplomacy, has grown frosty over the past 18 months. The Obama administration, though nervous about the ascendancy of political Islam, has also grown wary of the authoritarian methods being deployed by Egypt's generals.

In a further development, campaign officials for Ahmed Shafik, the last prime minister to serve under deposed President Hosni Mubarak, claimed yesterday that their man had won the presidential poll. Mohamed Morsi, the Brotherhood candidate, has also claimed victory. Official results are due to be released tomorrow.

Last night, further conflict loomed as plans for a "million man march" in Cairo's Tahrir Square emerged. The demonstration was called by the Muslim Brotherhood but was supported by secular revolutionary groups such as the 6 April youth movement, which played a major role in last year's uprisings. The move appears to show how the political sands are shifting again.

Liberal political factions have previously accused the Brotherhood of stitching up a closed-door deal with the military in exchange for political gains, and many still subscribe to this view.

But Tamir Fouad, a spokesman for 6 April, said he believed that the Brotherhood is now under no illusions about the military's alleged intentions.

"They think we were right about our campaign against the military council," said Mr Fouad. "I think now they believe in the revolution."

"The confrontation between the military and Muslim Brotherhood is going to intensify," said Hani Shukrallah, managing editor of Ahram Online. "The military is now in conflict with them on almost every single issue."